Nonaqueous electrolyte secondary batteries (hereinafter also referred to as a “nonaqueous secondary battery”) such as a lithium secondary battery are currently in wide use as batteries for devices such as a personal computer, a mobile telephone, and a portable information terminal.
A device equipped with a lithium ion battery includes a wide variety of electrical protection circuits, provided in a battery charger or a battery pack, so that the battery operates normally and safely. However, if a breakdown or a malfunction, for example, occurs in the protection circuits, the lithium ion battery may be continuously recharged. This may cause oxidative and reductive degradation of an electrolyte on surfaces of a cathode and an anode both of which generate heat, oxygen release caused by decomposition of a cathode active material, and even deposition of metallic lithium on the anode. This may eventually cause the lithium ion battery to fall into a thermal runaway. There is also a danger that ignition or explosion may occur in the lithium ion battery, depending on the situation.
In order to safely stop a battery before such a dangerous thermal runaway occurs, most lithium ion batteries currently include, as a separator, a porous base material which contains a polyolefin as a main component and which has a shutdown function of clogging pores present in the porous base material when a temperature inside the battery is raised due to some defect and reaches approximately 130° C. to 140° C. Exhibition of the shutdown function at a temperature rise inside the battery stops passage of ions in the separator, and thus allows the battery to safely stop.
Meanwhile, the porous base material containing a polyolefin as a main component is poor in adhesiveness with respect to an electrode. This may cause a decrease in battery capacity and a deterioration of a cycle characteristic. With the aim of improving the adhesiveness of the porous base material with respect to an electrode, there have been developed (i) a separator in which a porous layer containing a polyvinylidene fluoride-based resin is laminated on at least one surface of the porous base material and (ii) an electrode on a surface of which a porous layer containing a polyvinylidene fluoride-based resin is laminated.
For example, Patent Literature 1 discloses a separator arranged such that a porous layer, containing (i) inorganic particles as heat-resistant particles and (ii) a polyvinylidene fluoride-based resin as a binder resin, is laminated on a surface of a porous film.